The experimental observation, manipulation and utilization of coherent quantum mechanical properties in solid-state systems are key technological challenges for this century. The importance of incoherent quantum properties has been essential for the development of microelectronics and it is hoped that coherent quantum effects will spawn new technologies including, but not necessarily limited to, quantum computers [34].
There is at present limited experimental experience in coherent solid state systems without optical interactions, being mainly limited to superconducting systems of qubits [35,43,33] coupled 2 qubit systems [37,27] and 2D electron gas systems [47]. Although the successes of the superconducting work have been impressive, there is a strong impetus to develop coherent technologies that are compatible with the conventional semiconductor industry, due to the mature manufacturing technology and scalability advantages [28,32,24,9]. Although a truly coherent qubit is yet to be realized in a semiconductor system, incoherent precursors to qubits have already been fabricated, for example [41,8,6] and the rapid progress to date suggests that truly coherent effects will be observed soon.
An early suggested element for a quantum computer is the so-called ‘charge qubit’. The charge qubit is where a charged particle, usually an electron or Cooper-Pair, can be in one of two distinct spatial positions. With electrical control and quantum coherence, such a two-state system can be used as a qubit. The concept of a charge qubit as a scalable system for performing quantum operations in a solid-state environment goes back to early work by Ekert et al [13,2,14] and Landauer [31]. The attractiveness of such schemes is the relative ease of control and readout, and the obvious progression of such concepts from the incoherent control of conventional computing architectures. Control can be performed using gate electrodes, and readout via sensitive electrometers, for example single-electron transistors, SETs, or quantum point contacts.
One important system that has been suggested for the realization of a charge qubit is the P-P+ charge based quantum computer [24]. In this scheme, the qubit is defined by an electron localized to either the left or the right phosphorous ion, which constitutes two sites. This scheme has some interesting advantages over other, competing approaches, namely that readout is fairly easy to be achieved with single-electron transistors (SET) and gate operation time should be reasonably fast (˜50 ps).
A major problem with the charge scheme is the relatively high decoherence associated with distributions of charge. In fact the coupling which so readily provides the readout, is also responsible for the rapid dephasing. One method for ameliorating this problem has been suggested, namely operation in the so-called superposition basis, and preliminary experiments in superconducting systems are suggestive of significant improvements in dephasing being attainable [43]. In the superposition basis, one operates near a degeneracy point, so that the qubit is not defined by the charge being on the one site or the other, but rather by the symmetric or anti-symmetric combination of sites. Operation at this point has a greatly reduced sensitivity to noise as the potential landscape is reasonably flat.
A further problem related to the decoherence is that population decay can occur in position space qubits on timescales which may be short compared to the measurement time needed for single shot readout with an electrometer, see for example Buehler et al. [7].
Another reason for pursuing measurements in the superposition basis derives from the need to perform state tomography [25] in order to characterize qubit performance. In state tomography the entire density matrix of the qubit (or more generally of the qubit system) after gate operation is mapped out. It requires access to non-orthogonal bases to work, and therefore access to both the position and superposition bases is necessary.
Until now, however, there has not been a natural method for performing readout of the superposition states and it is this problem which is the subject of this patent. For clarity, we will focus the following discussion of the invention to the P-P+ paradigm for quantum computing, however it will be readily seen that the invention can be applied more generally.